When oligomerizing light olefins within a refinery, there is frequently a desire to have the flexibility to make high octane gasoline, high cetane diesel, or combination of both. However, catalysts that make high octane gasoline typically make product that is highly branched and within the gasoline boiling point range. This product is very undesirable for diesel. In addition, catalysts that make high cetane diesel typically make product that is more linear and in the distillate boiling point range. This results in less and poorer quality gasoline due to the more linear nature of the product which has a lower octane value.
The oligomerization of butenes is often associated with a desire to make a high yield of high quality gasoline product. There is typically a limit as to what can be achieved when oligomerizing butenes. When oligomerizing butenes, dimerization is desired to obtain gasoline range material. However, trimerization and higher oligomerization can occur which can produce material heavier than gasoline such as diesel. Efforts to produce diesel by oligomerization have failed to provide high yields except through multiple passes.
When oligomerizing olefins from a fluid catalytic cracking (FCC) unit, there is often the desire to maintain a liquid phase within the oligomerization reactors. A liquid phase helps with catalyst stability by acting as a solvent to wash the catalyst of heavier species produced. In addition, the liquid phase provides a higher concentration of olefins to the catalyst surface to achieve a higher catalyst activity. Typically, this liquid phase in the reactor is maintained by hydrogenating some of the heavy olefinic product and recycling this paraffinic product to the reactor inlet.
The products of olefin oligomerization are usually mixtures of, for example, olefin dimers, trimers, and higher oligomers. Further, each olefin oligomer is itself usually a mixture of isomers, both skeletal and in double bond location. This is also true of isomers in which access to the double bond is sterically hindered. Olefin types of the oligomers can be denominated according to the degree of substitution of the double bond, as follows:
TABLE 1Olefin TypeStructureDescriptionIR—HC═CH2MonosubstitutedIIR—HC═CH—RDisubstitutedIIIRRC═CH2DisubstitutedIVRRC═CHRTrisubstitutedVRRC═CRRTetrasubstitutedwherein R represents an alkyl group, each R being the same or different. Type I compounds are sometimes described as α- or vinyl olefins and Type III as vinylidene olefins. Type IV is sometimes subdivided to provide a Type IVA, in which access to the double bond is less hindered, and Type IVB where it is more hindered.
To maximize distillate produced in a refinery, refiners may contemplate oligomerizing FCC derived light olefins to make heavier oligomers, thereby shifting gasoline into the distillate range. However, not all refiners have cost advantaged FCC derived light olefin streams available. In some cases, light paraffins are the most cost advantaged feed.
Light paraffins are incapable of being converted to liquid fuels by oligomerization directly. Combination processes for the conversion of light alkanes to light alkenes, followed by oligomerization of the alkene to liquid fuels are known. U.S. Pat. No. 4,293,722 describes a process comprising dehydrogenation of propane followed by catalytic condensation to form C9 hydrocarbons, U.S. Pat. No. 4,304,948 describes a process comprising dehydrogenation of butane followed by catalytic condensation to form C8-C12 hydrocarbons, U.S. Pat. No. 4,542,247 describes a process comprising dehydrogenation of paraffins followed by 2 steps of oligomerization with an intermediate separation step, U.S. Pat. No. 6,897,345 describes a process comprising the isomerization of n-butane to isobutane followed by dehydrogenation and dimerization, U.S. Pat. No. 5,856,604 describes a process comprising dehydrogenation of isobutane, compression and oligomerization, U.S. Pat. No. 5,847,252 describes a process comprising low severity dehydrogenation of isobutane, oligomerization and saturation and U.S. Pat. No. 4,879,424 describes a process comprising feeding a heated feedstream to a zeolite catalyst to form olefins and aromatics and then passing this product stream to a second reaction zone where oligomerization occurs. These combined processes suffer from drawbacks such as low pressure oligomerization, aromatic formation or formation of high quantities of gasoline. Distillate range products are desired, particularly with a high cetane number.